Pohlsepia mazonensis, a visually underwhelming fossil from Illinois, fundamentally broke our understanding of cephalopod evolution. Described in 2000 and hailed as the oldest known octopus in the fossil record, the specimen dated back to the late Carboniferous period, roughly 311 to 306 million years ago. Pohlsepia was an outlier—all other fossil records strongly suggested that crown coleoids, the group containing octopuses, squid, and cuttlefish, diverged much later, during the Jurassic.
To solve this puzzle, Thomas Clements, a paleontologist at the University of Leicester, and his colleagues put this supposed oldest octopus fossil through a series of high-tech imaging tests. They found Pohlsepia was not an octopus at all. Instead, it was a decomposed, squashed nautiloid.
A Rorschach test
The reason a nautiloid managed to masquerade as an octopus for almost a quarter of a century was due to the way that fossils from the Mazon Creek Lagerstätte formed. Around 300 million years ago, this area was a brackish, tidal marine basin that was periodically inundated by massive amounts of iron-rich river mud. When organisms died and were buried in this sediment fan, the high iron content triggered the precipitation of the mineral siderite around their decaying bodies, locking them inside hard geological nodules.
